1. Field of the Invention
The invention generally relates to a crank arm, and in particular, to a bicycle crank arm with an increased contact area and having a shear force resisting function.
2. Related Art
A crank arm is provided on a bicycle and is mainly for transmitting the the rider's stepping force into a driving force to drive the bicycle move forward, so the crank arm must have enough strength to resist the shear force derived during the pedaling. Therefore, for a conventional bicycle crank arm, metal material is the first choice to make a crude unfinished product through casting and then surface processing and treatment are performed. However, as industrial technologies develop, bicycle parts are required to be lighter in weight, and much safer during a violent ride. Gradually, choosing aluminum alloy or composite metal material as the material to produce a crank arm becomes a tendency. Even though there are many forging methods used for processing an interior of the crank arm to make the crank arm hollow, thereby achieving the purpose of reducing weight. However, to manufacture a hollow crank arm through forging methods is troublesome and time-consuming and now becomes outdated for a long time ago.
In public patent applications, such as a Taiwan patent application “Bicycle Crank Arm Structure”, the patent publication number of which is 431405. A crank body 1 and a cover plate 2 integrally formed by adopting an aluminum alloy are disclosed. The crank body 1 has a head and a tail opposite to each other in a longitudinal direction, and a sleeve joint hole 3 opened at the head for locking a through tube of a bicycle. An opening part 4 is formed on a rear bent part adjacent to the head on the crank body 1, and a slot hole 5 is formed inside the crank body 1. One end of the slot hole 5 is a closed end, and the other end of the slot hole 5 communicates with the opening part 4. At least one reinforcing rib 6 is provided inside the slot hole 5 to enhance the strength of the crank arm. The cover plate 2 is welded or fixedly covered to the opening part 4, so that the crank body 1 is hollow, thereby achieving the objectives of reducing the weight and lowering the cost.
However, it is difficult to dig the slot hole 5 inside the crank body 1. In particular, the slot hole 5 has a plurality of reinforcing ribs 6 therein, so that a space for processing in the slot hole 5 is rather small, and operations are difficult. After the slot hole 5 is formed through processing, the cover plate 2 must be sealed over the opening part 4 in a welding manner, which is very troublesome and time consuming.